Polyakovite-(Ce), ideally REE 4 Mg Cr 3+ 2 Ti 2 Si 4 O 22 , monoclinic, a 13.398(1), b 5.6974(5), c 11.042(1) Å,  100.539(2)°, V 828.6(2) Å 3 , a:b:c = 2.3516:1:1.9381, space group C2/m, Z = 2, is a new mineral species from the Ilmen Mountains, southern Urals, Russia. It is of hydrothermal origin; associated minerals are calcite, dolomite, fluororichterite, phlogopite, forsterite, monazite-(Ce), clinohumite, chromite and davidite-(Ce). It occurs as irregular grains up to 3 cm in diameter and euhedral crystals elongate along [010] (up to 2 mm) and flattened on {001}. The main forms are a{100}, c{001}, t{201}, {201}, m{110}, o{111}, n{111}, p{112}, {112} and x{302}. It is translucent in thin fragments, black with a brown streak, has a vitreous luster, and does not fluoresce under ultraviolet light. Polyakovite-(Ce) has a Mohs hardness of 5½-6, is brittle with a conchoidal fracture, and has no cleavage or parting. The measured density is 4.75(7) g/cm 3 , D calc is 5.05 g/cm 3 . It is highly metamict. The infrared absorption spectrum has maxima at 473, 970 and 1115 cm -1 . In reflected light, polyakovite-(Ce) is gray, non-pleochroic, isotropic, n in the range 1.931-1.935. The strongest five reflections in the X-ray powder-diffraction pattern of annealed material [d in Å(I)(hkl)] are: 2., sum 99.75 wt.%; the Fe 2+ /Fe 3+ ratio was derived by Mössbauer spectroscopy. The corresponding chemical formula is (Ce [based on O = 22 apfu (atoms per formula unit)]. The mineral is named for Vladislav Olegovich Polyakov , who contributed greatly to our knowledge of the mineralogy of the Urals.The crystal structure of polyakovite-(Ce) (annealed single crystal) has been solved by direct methods and refined by leastsquares to an R value of 4.6% using 1074 unique observed (|F o | > 4F) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoK␣ X-radiation. Polyakovite-(Ce) is a sorosilicate of Cr 3+ , Ti 4+ , Fe 2+ and REE 3+ , and the first titanosilicate where Cr 3+ is a species-forming element. In the crystal structure of polyakovite-(Ce), there are four octahedrally coordinated M sites: M(1), 0.80 Mg + 0.20 Fe 2+ , 2.108 Å; M(2), 1.52 Ti + 0.32 Nb + 0.16 Ⅺ, 1.984 Å; M(4) and M(3) are occupied by 0.64 Cr 3+ + 0.36 Fe 3+ , 1.995 Å. Two distinct Si sites are occupied solely by Si, and the resulting (SiO 4 ) tetrahedra share one vertex to form an [Si 2 O 7 ] group: 1.614 Å, Si(1)-O-Si(2) 173.2(8)°. There are two types of chains of octahedra: (1) [M(3) and M(4)] octahedra, and (2) M(2) octahedra, parallel to the b axis, form together an octahedral layer parallel to (001). In a chain, every two octahedra have a common edge. M(2-4) octahedral layers, M(1)